SE505980C2 - Ways to prevent peroxide-degrading enzymes by bleaching with hydrogen peroxide - Google Patents

Abstract

PCT No. PCT/SE94/01245 Sec. 371 Date Aug. 8, 1995 Sec. 102(e) Date Aug. 8, 1995 PCT Filed Dec. 22, 1994 PCT Pub. No. WO95/17546 PCT Pub. Date Jun. 29, 1995A method and composition for suppressing or inhibiting the decomposing action of enzymes on hydrogen peroxide, e.g. peroxidase and catalase, during bleaching of cellulose fibres with hydrogen peroxide, especially in connection with the production of recycled paper, in such a way that microorganisms are not markedly affected, and discharges that are dangerous to the environment are minimized. The composition contains hydroxylamine, thiocyanate salts, formic acid, ascorbic acid or nitrites.

Description

505 9so 2 It has also been proposed to inhibit the action of the catalase by raising the temperature to 60-90 ° C or the pH value to 10-14. An increase in temperature means that the hydrogen peroxide is degraded, and an increase in pH causes the mass to yellow and the need for bleaching increases or the mass breaks down.

Addition of biocides has also been proposed to kill the microorganisms and prevent catalase formation. Biocides are hazardous to health and unsuitable from an environmental point of view.

Metallic ions present, such as iron, manganese, copper and aluminum, break down hydrogen peroxide. To prevent this, chelating agents and sodium silicate are currently used. Silicates tend to clog the equipment during pulp handling and the complexing agents are expensive and sometimes dangerous to use. Their impact on the environment is strongly questioned and not investigated.

From EP 0 562 835 it is known to use small amounts of chlorine dioxide, bromine, chlorine, iodine and ozone as oxidizing, enzyme inactivating and bactericidal agents in bleaching cellulose pulp with hydrogen peroxide. According to embodiment 3 of the description, the microorganisms in dead water are killed with chlorine dioxide. Chlorine dioxide is a toxic, explosive, corrosive and difficult-to-handle substance, which decomposes easily and must be manufactured in the bleaching plant and stored there diluted.

G. Galland, E. Bernard, Y. Verac propose in "Achieving a de- inked pulp with high brightness" in Paper Technology, December 1989 various ways to increase the brightness in the processing of waste paper by reducing the consumption of hydrogen peroxide. Among other things, it is proposed that catalase be removed by eliminating the microorganisms that develop the enzyme or that catalase be destroyed before bleaching. To destroy catalase, e.g. a treatment with sodium hypochlorite. However, sodium hypochlorite is an unsuitable substance for the environment.

In principle, the procedures according to the two publications involve using the environmentally hazardous bleaches, which are now 505 980 3 trying to avoid, in smaller amounts to kill the microorganisms and inactivate the enzyme.

According to the present invention, there is proposed the use of a substance according to claim 1, which suppresses or inhibits the degrading action of enzymes, preferably peroxidases, such as catalase on hydrogen peroxide in bleaching cellulosic fibers with hydrogen peroxide, which substance does not kill microorganisms.

In particular, compounds are used which are not environmentally hazardous and which can therefore be handled without additional measures during use and then discharged into the sewer without adversely affecting any subsequent biological treatment before the water is discharged into the recipient. Substances which do not consist of or form environmentally hazardous halogen-containing molecules are preferred.

These substances also have no negative impact on modern biocide-free mucus control systems (Biochem®, Bimogard®), which are increasingly being used in the cellulose and paper industries. However, unlike biocides, they are expected to have a beneficial effect on this type of system.

Hydrogen peroxide can penetrate the cells of microorganisms and be broken down internally by enzymes, especially catalase. The breakdown also takes place outside the cells by secreted enzyme such as catalase. The amount of catalase outside the cells increases if the cells are killed and ruptured. It is therefore assumed that the optimal inhibition of the degrading effect of the catalase takes place by inhibiting the catalase present outside the cells and at the same time ensuring that the microorganisms do not die and burst.

It is believed, without limiting the invention to this, that the inhibitory effect of the substances arises by blocking or destroying the active center on the enzyme molecule. The active center of catalase contains chemically bound iron which can be blocked and inactivated by complex formation. The substances can also inhibit enzymes other than catalase, such as peroxidases. Compositions containing at least one of the following substances can be used according to the invention: Hydroxylamine, salts and addition salts thereof such as hydroxylammonium sulphate and chloride; thiocyanate compounds such as ammonium thiocyanate; formic acid, ascorbic acid. Nitrite salts of alkaline earth metals are also referred to. Mixtures of these compounds can also be used. As mentioned, environmentally hazardous halogen-containing molecules are avoided. However, some halogen salts are not environmentally hazardous at the concentrations used.

Preferably, compositions containing non-oxidizing substances such as hydroxylamine, salts and addition salts thereof such as the hydroxylammonium sulfate and chloride are used; ascorbic acid, and mixtures thereof, in particular hydroxylammonium sulphate.

The enzyme inhibitors according to the invention can be used in bleaching all types of cellulose pulps e.g. such made by the sulphite and sulphate methods, mechanical pulp, thermomechanical or chemithermomechanical pulp or pulp made by re-wrapping recycled paper. Since the catalase-inhibiting substances according to the invention do not kill microorganisms, they may also be suitable for bleaching with hydrogen peroxide of pulps produced by microbiological lignin degradation.

The invention also relates to a process for the treatment and hydrogen peroxide bleaching of cellulose, wherein at least one composition according to the invention is used.

The enzyme inhibitory substance or composition of the invention can be applied anywhere during the manufacture of the bleached pulp. Thus, the composition or substances can be added anywhere during the treatment of the pulp, such as in the pulp production itself before the pulp is bleached or anywhere during the bleaching itself. The invention relates in particular to a process for bleaching recycled paper, in which paper, water, chemicals such as alkali silicates, surfactants such as surfactants, and optionally complexing agents are suspended, the suspension is purified and pressed into a pulp, the pulp is bleached with hydrogen peroxide and purified from ink by flotation, optionally also with cyclones, vortex cleaners and screening and filtration, which process is characterized in that at least one catalase inhibitor is added before the bleaching and possibly in backwater from the various treatment steps and in that the flotation is preferably carried out in the presence of hydrogen peroxide.

By backwater is meant separated liquid, which is returned to the previous step, especially to the first so-called storage step where paper, water and chemicals are mixed.

Bleaching with hydrogen peroxide is usually carried out at alkaline pH, and the pH value is usually above 5, preferably above 8, for example between 7 and 12, in particular between 8 and 11.5.

The amount of inhibitor required can be determined by those skilled in the art after determining the residual peroxide content of the streams and pulp during processing and taking into account the brightness of the pulp. It depends on the pulp used, in particular when using recycled paper, which may contain varying amounts of microorganisms depending on the storage conditions. The amount of enzyme-inhibiting substance added is generally between 0.01 and 1.5% by weight, calculated on dry solid mass (dry weight), preferably 0.01-0.5, in particular 0.01-0.05% by weight. %. In some cases, significantly larger quantities may be needed.

Alternatively, the addition can be made with 0.5-110% by weight, preferably 5-50% by weight of enzyme-inhibiting substance calculated on the amount of hydrogen peroxide charged. The larger amounts are added when an extra large amount of inhibitor is needed, when the hydrogen peroxide content has dropped.

The invention is described in more detail with reference to the accompanying figures, of which Asos 9bo 6 Fig. 1 shows a block diagram of a bleaching process with return- PaPPerf Fig. 2 shows diagrams of the hydrogen peroxide content in the flow out of the bleach tower 6 (curve A) and the rear water tank 20 (curve B) in Fig. 1 in the practice of the invention in bleaching recycled pulp according to Embodiment 1, Fig. 3 shows the same kind of diagram as in Fig. 2 but with respect to Embodiment 2, Fig. 4 shows the degradation of added hydrogen peroxide when testing catalase-containing water from Thun presses 14 in Fig. 1 during normal operation without the inhibitor according to the invention when this is provided with O (curve H) and 1 mmol / l (curve G) hydroxylammonium chloride, Fig. 5 shows curves corresponding to Fig. 4 but with the addition of O (curve Q), 1 mmol / l hydroxylammonium chloride (curve O) and 0.5 mmol / l of each hydroxylammonium chloride and ascorbic acid (curve P), Fig. 6 shows tests of hydroxylammonium sulphate (curve A), ammonium thiocyanate (curve C) oc h formic acid (curve D) in comparison with iodine (curve E) and on prepared catalase solution in the presence of hydrogen peroxide.

The inhibitors according to the invention have a better effect than the known inhibitors in bleaching cellulose fibers with hydrogen peroxide. Because they do not have a biocidal effect and do not kill the microorganisms at the concentrations used, they do not upset the balance of the microflora in the pulp and do not have a detrimental effect on the environment during emissions, e.g. in treatment plants. The fact that microorganisms do not die means that the catalase concentration is lower in the mass. The proposed inhibitors are economically advantageous and can be handled during operation without special measures. The use of the inhibitors means that it is easier and more economical to increase the hydrogen peroxide concentration during flotation. It then becomes more efficient. The invention will now be described by means of non-limiting exemplary embodiments. Weight indications refer to dry-mass (dry matter).

Example 1. Experiment with hydroxylammonium sulphate as inhibitor in a waste paper treatment plant.

Hydroxylammonium sulphate has been tested as a catalase inhibitor in a waste paper treatment plant. Figure 1 schematically shows how recycled paper is processed in the plant.

In a mixer 1, recycle paper, water, sodium hydroxide, hydrogen peroxide and collectors (Raisapon EM dispersion of fatty acid salts) are added. The resulting pulp suspension is purified and then pressed using a coarse screen 2, turbo-separator 3 and reject-sorting cyclones 4. After the belt press 5, complexing agents (Aktiron magnesium salt of a nitrogen-based complexing agent), hydrogen peroxide and sodium hydroxide are added and the pulp is further purified in the belt presses 5. After the belt press 5 more hydrogen peroxide is added and then the pulp is bleached in a bleaching tower 6 and cleaned with sand cyclones 7, primary and secondary flotation 8 and 9 and vortex cleaners 10 and 12 with intermediate silage 11, disc filters 13, Thun presses 14, kneading machines 15 (Frota pulp) and fed to storage tower 16.

A part of the process liquid separated from the belt presses 5 is fed to the microflotation plant 17 and forms baking water I, which is returned to the storage mixer 1. From the microflotation plant 17 separated material is fed to a centrifuge 19 after the secondary flotation 9, and the liquid after the centrifuge 19 is returned to the microflotation plant 17.

Liquid leaving the disc filter 13 is fed to a rear water tank 20, to which press tray water is also fed. From the rear water tank 20, rear water II is fed to the rear water tank 18. In such a plant, an experiment was carried out for 44 hours with the addition of hydroxylammonium sulphate in the form of a 10% by weight aqueous solution. The inhibitor solution was added before bleaching tower 6 in a mixing vessel (not shown), followed by a mixer (not shown). This addition point was chosen because the pulp before the mixer has a high dry content in the system and thus a high content of inhibitor is obtained in the pulp before the addition of hydrogen peroxide in bleaching tower 6. The dosage amount was at the start 0.113 kg / ton dry-dried pulp but was first increased to 0.254 kg / ton and then to 0.339 kg / ton at the end of the test period (Dsl, Ds2 and Ds3 in Figure 2). At the start of the experiment, the dosage of hydrogen peroxide was 11 kg / ton pulp (1 kg in the storage mixer 1 and 10 kg in the mixer for bleaching tower 6).

After 10 hours, the additive in the storage mixer 1 was switched off and after 30 hours the additive in the mixer was reduced to 9 kg / ton mass. The experiment was stopped after 44 hours.

The hydrogen peroxide content was determined by the method described in Vogel, Artur I .: Vogel's textbook of quantitative chemical analysis, fifth edition, page 394.

Figure 2 shows in curve A the concentration of hydrogen peroxide in mg / l in the flow out of the bleaching tower 6 during the experiment. Curve B shows the hydrogen peroxide concentration in the rear water tank 20. From the 14th hour until the end of the experiment, the concentration of hydrogen peroxide was measured before and after the primary flotation 8. Before the flotation the concentration was about 100 mg / l and after the flotation about 80 mg / l until hour 33 and decreased during the last hours to about 60-70 mg / l.

The experiment was performed so that the mass lightness remained within the normal variations.

Example 2 Another experiment was performed in the plant described in Example 1. 505 980 9 The experiment was performed for 95 hours. Hydrogen peroxide was not charged in the storage mixer 1, and in the mixer before the bleaching tower 6 the amount was reduced from 10 to 8 kg / ton of dry solid mass. The weight percent aqueous solution of hydroxylammonium sulfate was added at start at 60 l / h (Dsl). After 41 hours it was found necessary to increase the dosage in the mixer to 70 l / h (3.96 kg / ton) and also to add 10 l / h (0.55 kg / ton) to the filtrate from the belt presses 5 (Ds2) . This was done to counteract an increased catalase activity in the backwater I. After 49 hours, the activity in the backwater I circulation decreased, so this addition was discontinued (Ds3). The catalase activity then increased, so that the dosage was reintroduced {Ds4).

Figure 3 shows in curve A the concentration of hydrogen peroxide after the bleaching tower 6 and curve B in the rear water tank 20. The amount of residual peroxide has during the test run period been at an even level at about 80 mg / l at a dosage of 10% inhibitor solution of 60 l /hrs. Even in this case, the mass brightness was within the normal variations.

The experiment shows that the dosage of hydrogen peroxide can be reduced by 3.5 kg / ton pulp or about 20-30%. The amount of inhibitor used was about 200-400 ppm based on dry mass.

Better purification was obtained during the flotation.

Example 3. Catalase inhibitory effect of hydroxylammonium chloride In another sample, hydroxylammonium chloride was added to a concentration of 1 mmol / l to a sample from the microflotation 17 in a plant according to Fig. 1. The operation took place without the addition of inhibitor according to the invention and the experiment was carried out in the same manner as in Example 3. The result is shown in the diagram in Fig. 4 where the percentage of residual peroxide is indicated as a function of the time after hydroxylammonium chloride has been added. Curve G shows the residual concentration of hydrogen peroxide after addition of hydroxylammonium chloride and curve H without any addition.

After one hour, about 75% of the hydrogen peroxide remains. Example 40 Catalase inhibitory effect of hydroxylammonium chloride and ascorbic acid To investigate the effect of an addition of hydroxylammonium chloride in admixture with ascorbic acid, a mixture of equal parts (0.5 mmol / l) of hydroxylammonium chloride and ascorbic acid was added. and in a second experiment only hydroxylammonium chloride (1 mmol / l) concentration. The experiment was carried out on water from the Thune presses as in Example 3. The diagram in Fig. 7 shows the residual peroxide with the addition of the mixture with the curve P and with the addition of only hydroxylammonium chloride with the curve O, without the addition of catalase inhibitor is indicated by the designation Q.

Example 5. Analysis of catalase inhibitory effect of formic acid, hydroxylammonium sulphate and ammonium thiocyanate on standard solution of hydrogen peroxide.

The catalase inhibitory effect of some substances was measured on a standard solution of hydrogen peroxide.

A 0.025M buffer solution of KHJKL and Na fl ïh was prepared. mg of catalase was dissolved in 100 ml of the buffer solution. The catalase was from Sigma with an activity of 2000 units per mg protein, one unit decomposes 1 pmol hydrogen peroxide per minute at pH 7 and 25 ° C when the hydrogen peroxide content goes from 10.3 to 9.2 mM). ml of the catalase buffer solution and 1 mmol of inhibitor are mixed with 180 ml of pure buffer solution and 2 ml of 10% hydrogen peroxide are added. Hydroxylammonium sulfate, ammonium thiocyanate and formic acid were examined. 50 ml samples were taken after 5 and 15 minutes, respectively, to check the remaining amount of hydrogen peroxide.

The result is shown in Figure 8, where curve A shows% residual peroxide when testing hydroxylammonium sulphate, curve C when testing ammonium thiocyanate and curve D when testing formic acid. It appears that these compounds are stronger catalase inhibitors than iodine (curve E) according to EP 0 562 835.

Claims (10)

10 15 20 25 30 35 40 505 980 11 Patent claims Use of a composition containing at least one substance selected from: a) hydroxylamine and salts and addition salts thereof, such as the sulphate and chloride; b) thiocyanate salts, such as ammonium thiocyanate; c) formic acid; d) ascorbic acid; and e) nitrite salts of alkaline earth metals; to inhibit hydrogen peroxide degrading enzymes, such as catalase in connection with the preparation and / or bleaching of cellulose fibers with hydrogen peroxide. Use according to claim 1, characterized in that the composition also contains complexing agents and / or detergent. Use according to claim 1 or 2, characterized in that the composition contains hydroxylamine, hydroxylammonium sulphate or hydroxylammonium chloride. Use according to any one of the preceding claims, characterized in that the cellulose fibers are included in a cellulose pulp made from recycled paper. Process for the treatment and / or bleaching of cellulose with hydrogen peroxide, characterized in that a composition containing at least one substance selected from: a) hydroxylamine and salts and addition salts thereof, such as the sulphate and the chloride; b) thiocyanate salts, such as ammonium thiocyanate; c) formic acid; D) ascorbic acid; and e) nitrite salts of alkaline earth metals; and optionally f) complexing agents and / or detergents are added in such amounts that the action of hydrogen peroxide-degrading enzymes, such as catalase, is inhibited. Process according to Claim 5, characterized in that the amount of enzyme-inhibiting substance added is between 0.01 and 1.5% by weight, calculated on dry-mass, preferably 0.01-0.5% by weight. Process according to Claim 5, characterized in that 0.5 to 110% by weight, preferably 5 to 50% by weight of enzyme-inhibiting substance is added, calculated on the amount of hydrogen peroxide charged. Process according to Claim 5, characterized in that hydroxylammonium sulphate is added in an amount of 0.01 to 1.5, preferably 0.01 to 0.5% by weight, based on dry-thinking cellulose pulp. Process according to one or more of Claims 5 to 8, characterized in that the enzyme-inhibiting substance is added anywhere during the mass production or bleaching, preferably before the bleaching and optionally to backwater which is returned to bleaching. Process according to one or more of Claims 5 to 8, characterized in that the enzyme-inhibiting substance is added to improve the flotation of impurities.